The recirculation of some engine exhaust gas through an exhaust gas recirculation (EGR) loop to mix with charge air flow to the engine cylinders can aid in controlling tailpipe emissions, especially NOx (oxides of nitrogen) and particulates. Commercially acceptable diesel engine performance in a motor vehicle, especially a commercial vehicle, compels the use of supercharging. Hence, two-stage turbochargers find significant use in large vehicles powered by diesel engines. It is known to control EGR in such an engine through a high-pressure EGR loop in which some of the exhaust gas from the engine cylinders is diverted away from the high-pressure turbine stage of the turbocharger and recirculated through the high-pressure EGR loop to a mixer where it mixes with charge air coming from the high-pressure compressor stage that is being powered by the high-pressure turbine stage. The mixture of recirculated exhaust gas and charge air then passes into the intake manifold and eventually the engine cylinders.
Because a diesel engine that powers a motor vehicle runs at different speeds and loads depending on various inputs to both the vehicle and the engine that influence engine operation, exhaust gas recirculation requirements change with engine speed and load changes. A processor in an engine control system processes data indicative of parameters such as engine speed and engine load to develop control data for controlling various aspects of engine operation including the quantity of exhaust gas being recirculated.
While increasing the percentage of EGR (i.e. increasing the EGR rate) in the charge air can be effective to promote better reduction of such emissions, it is public knowledge that a majority of manufacturers of turbocharged diesel engines, possibly out of concern that certain engine operating conditions (for example, low engine speeds and high engine torques) may not provide sufficiently large EGR percentages, are pursuing tailpipe emission control strategies other than EGR control strategies in an effort to qualify engines for compliance with applicable governmental regulations for tailpipe emissions.